It's hard to make Twenty One Pilots fit in any of the traditional boxes. In fact, trying to explain them usually starts with a list of instruments and costumes. There's two of them: one drums, the other one plays piano, raps, plays ukulele, and sings about the human condition. It gets seriously heavy at times, but there's always a strong, party synth hook just around the corner.

They've been a thing for a little while, building up a significant fanbase first in their home state of Ohio before catching the attention of their label, Fueled By Ramen. Their album Vessel was released last year, and tour support slots with Fall Out Boy and Paramore have quickly gathered them some serious fan-harvesting momentum.

Selling out the majority of this, their first UK tour proper (they played a handful of low-key shows last year), the crowds that greet them are beyond psyched. Having queued all day in the cold and rain, decked out in a uniform of Twenty One Pilots t-shirts, skeleton costumes and balaclavas, the venue is almost full by five minutes past doors.

They put on a performance worthy of the hype, too: a dazzling light show adding extra drama to an intense and quite ridiculous set. Tyler is rarely still -- and on the rare occasions he is, there's usually a devastating lyric being delivered from behind the piano. When Josh isn't drumming, it's because he's backflipping off the piano.

For all their on-stage theatrics, though, they're a pretty introverted pair. Taking a crew around with them consisting of talented mates from back home, they don't really need to make new pals. There's far more chance of finding them playing Flappy Bird than heading down the boozer after the show. Their fridge is filled with a strange British energy drink they haven't quite sussed out yet (Lucozade) and, although there are other members of their crew in the room for the interview, they observe a careful silence as they go about their business.

You’ve spent the past two weeks touring around mainland Europe. How’s that been?

Josh: Europe’s been really fun. It’s cool to be in countries and cities that are older than our own country, and walk around and see the history of each place and learn about more history, and think back to times that we’ve learned about certain things. Like in elementary school, even, and getting to see those things in real life. I just saw the Eiffel Tower for the first time the other day, and the Berlin Wall. Those are things that we grew up seeing on TV or on movies or documentaries or things at school…

Tyler: Or on the backgrounds of computers.

Josh: Exactly, like computer wallpapers or calendars. So it’s really good, and it’s fun to be able to do this as a job - get to travel and see new places. I love it.

What have been the highlights?

Tyler: Playing in front of crowds that speak different languages is a really cool feeling.

What country or city has stood out?

Tyler: Paris was crazy.

Josh: I think Paris might have been one of my favourite shows.

Tyler: Well, there’s a bittersweet feeling towards Paris, though, because we also got some stuff stolen from us in Paris. Which was frustrating.

Has your fanbase turned out how you expected when you first started out?

Tyler: I don’t think we knew what it was going to turn out like or who was going to come out to shows. It’s cool to see a similarity… There’s definitely a type of person that comes out to our shows, but it’s not really specified in any sort of age or part of the world. But it’s the type of person who’s looking for an experience and something to hold on to, and those people, we meet them where they’re at. And I think that’s what live music is all about.

Do you think those people connect because they’re like you?

Tyler: Nobody’s like me.

Josh: Everybody’s like me.

Tyler: Everybody’s like Josh. (laughs) That’s a horrible thing to say! No, there’s some similarities. There’s lyrical content that talks about some stuff that people can resonate with.

You seem to have become much more confident and at ease in interviews over the past year or so. Have you felt that change?

Josh: I’m nervous right now. I still get nervous.

Tyler: I’m shaking in my boots. I started wearing boots, which is new. I think with wearing boots comes the confidence that you get when you buy a pair of boots. I used to not be about boots, but then I was tired of my feet getting wet and cold, so I bought some boots. And now I feel like I bought boots for my soul. I can now step on the things in life that need to be stomped upon. Which gives me confidence. Do you know what I’m saying, Josh? Give me an amen!

Josh: Amen, Tyler! I do! I remember three years ago, in the winter time, you had a pair of snow boots, and you were just like, you owned the neighbourhood. You were stomping on snow piles and I was like, that guy’s confident in his boots. I actually think, going back to your question, in High School, if I was in front of a group of people and somebody asked me a question, I would borderline throw up or pass out. I guess I’ve gotten a bit more used to talking in front of people or bring in front of people. I’m naturally more introverted. But I think doing it just helps. And having boots.

You’ve done some huge arena support slots - which do you prefer, those or your own shows?

Tyler: Being an opening band and trying to impress people who don’t know you are is a very tiring thing. It’s also very… playing in front of your own audience is very inspiring, and to say that being an opening band is not inspiring isn’t exactly what I’m going for, but it’s just not as inspiring as playing in front of your own crowd. Being an opening band and learning from bands that have gone before you is important. We’ve drawn a lot from the bands we’ve opened for - what to do, maybe sometimes what not to do. But I would much rather do our own show every day, to a small group of people than play in front of millions of people opening up, for sure.

Are there any bands you’ve not supported that you’d really like to?

Tyler: No.

Josh: Probably not.

Earlier on in my hotel, I could hear Celine Dion blaring from out the window, and at that moment I suddenly remembered having watched an interview where you spoke about Celine.

Tyler: We would like to open up for Celine Dion. We really would. I think that we could learn a lot from her. But more than her, probably her band.

Josh: Yeah, and her lighting director, I think. Inspiring.

I don’t think I’ve seen Celine’s light shows.

Tyler: If you’ve seen her, you’ve seen her light shows.

I’ve not seen Celine, I’ve got to admit.

Josh: Well, they make her look really good. Those lights make her look really good. She’s getting a little bit older…

Tyler: And so are we, which is why we are interested in the lighting that preserves age.

Josh: I think the reason we’re a little more inspired by her band is, I just don’t like her with short hair. It’s nothing to do with her musically, but just, her short hair isn’t as good as her long hair.

Tyler: And her whole band has long hair.

Josh: The band has long hair because they’re trying to protest short hair. And by protesting short hair, naturally, it just grows.

Tyler: Yeah. And you play better.

So will you be growing your hair out?

Josh: Yeah.

Tyler: Duh!

Josh: That’s what we’re really working on.

Man, can you imagine how good you’ll be then?!

Tyler: We’ll be so much better…

Celine has a waterpark in her garden. Is there anything crazy you’d like to own someday?

Josh: Oh my goodness. I haven’t thought about it too much, because I guess I don’t feel super materialistic. I think naturally everybody wants to have cool things, but… I would like to have at least one exotic animal. Like a… well… maybe not exotic…

Tyler: A MONKEY!

Josh: I was going to say a monkey.

Tyler: I’m sitting here waiting for you to say it.

Josh: That’s what I was moving towards!

Tyler: I know, but sometimes, something that important just needs to be said. You don’t beat around the bush. You know what also I would like for my monkey? I would like to have a hammock where I keep a ton of bananas, so when people come over, I go, ‘Do you want to see my banana hammock?’ And they’re not sure what I mean by that at first, but then I go and show them and say, ‘listen, I have a monkey, and I keep all his food in this big hammock in the back yard.’

Josh: But without knowing what you mean, they’re going to say yes, no matter what.

Tyler: And then you’d be like, ‘Dude… that’s wrong. Do you even know what a banana hammock is?’

Tyler: Our key messages are, it’s truly a conversation between a person and something that is bigger than themselves. Whether you want to call that the night sky or the eternal abyss of darkness, or some being that is God, really trying to talk to someone who has to know all the answers because we don’t. A lot of the things the content is talking about, is really just trying to figure out a lot of the important questions. I think that’s what wrong. Kids ask themselves the important questions and they don’t have the answers, and that’s when the struggle starts. I think that, as people get older and turn into adults and settle down into their life, they start to not ask themselves those questions any more. Which is why I think it’s cool to take this time in our lives to really try and figure out why we’re here and what we’re doing and what the point is. A lot of the content is about those questions, and it can get pretty heavy. But as dismal as it may seem at some points, I always like to make sure that there’s a shred of hope, at the end or in the bridge.

You’ve said in interviews that you are still searching for your purpose. Isn’t it doing this, and saying these things to help other people deal with them?

Tyler: Purpose is a huge thing, and whether or not you understand what yours is, what I’ve learned from our experience, as we see more and more success as a band, that if you look at someone and they seem successful or established, don’t confuse that with purpose, because I think that people still struggle, even when things are going really well. A lot of the same struggles and questions that we’ve had have not been answered just because we’re able to play shows around the world now. And I’m glad all of the answers aren’t there, I’m glad the questions aren’t solved, because then there would be no more songs to write.

Your fans share and create a lot in response to your songs. Is that something you expected?

Tyler: I don’t think we expected it, but we’re realising that the songs, or the content in them, warrant a reaction or reply. Whether that be artistically or just words, a lot of the people who support us and are a part of this thing, they hear what we’re talking about and they feel just compelled to share how it’s helped them, or how they resonate with what’s going on. So, at this point, we embrace the fact that it’s a back and forth conversation. It’s not just us spitting out ideas and leaving. We get to witness a lot of those people coming back and either wanting to show support or wanting to ask questions or wanting to share art or whatever it is. And I think that’s a great thing, because our crowds that come out to shows are filled with people who have something to say. That’s a cool moment.

What’s the most rewarding part of doing all of this?

Josh: I think it’s just that. Getting to hear people’s stories and the way that they interpret things or have used it in their own way, to either do something creative or something to maybe distract their mind from going to the dark places that all of ours naturally do. I know that growing up, and even now, I have things in my life that are very strategic distractions to try and stay away from going certain places, in my mind or even in the world. I remember when I first started finding some of those things, and maybe even seeing other bands or types of music that helped me with that, when I started becoming interested, before I could play, I always wanted to be a part of something that could also maybe be a distraction. To hear some people resonating with our music and what we say, I think that’s the most rewarding part of it. And getting to go to the Eiffel Tower, too.

You’ve got some pretty serious hits on YouTube. Have you got any other hot YouTube tips?

Tyler: Yeah, you should look up ‘baby laugh’ on YouTube. Also ‘soldier comes home’. Search that and there’s like, soldiers coming home, and babies and mums and kids crying. It’s just a great experience.

Josh: Ah yeah, that’d be heartwarming. I haven’t been on YouTube in a while. ’Twenty one pilots live’?